An electronically controlled automobile engine is generally provided with a sensor for detecting the rotating state of the engine.
As a kind of such sensor, a crank angle sensor for detecting the specific rotating position of the engine crankshaft is disclosed for example in Jikkai Sho. 58- 19532 published in 1983 and Tokkai Sho. 63-183375 published in 1988 by the Japanese Patent Office.
The crank angle sensor detects only a specific rotating position a so called reference position, but it may be desired to detect continuously the rotation angle of the crankshaft in a specific range from the reference position. Such necessity arises, for example, when determining misfiring of engine. In order to determine misfiring in a cylinder, the angular velocity of the crankshaft in the specific range from the reference position corresponding to the cylinder is detected, then the detected angular velocity is compared with the previously detected angular velocity in the same range or with the angular velocity detected in other specific range. When a speed drop more than a predetermined value is found in this comparison, the corresponding cylinder is determined to have misfired.
To obtain an angular velocity, the rotation angle of the crankshaft must be detected continuously, which is realized, for example, by detecting passing of teeth of a ring gear mounted on the engine crankshaft by a magnetic sensor, and counting the number of passing teeth.
The ring gear is a gear for starting up the engine by engaging with a pinion of a starter motor when starting up the engine. The magnetic sensor is provided oppositely to the teeth of the ring gear, detects changes of its own magnetic field by passing of teeth of the ring gear, and outputs a pulse signal. Therefore, by counting the pulses of the signal after output of the reference position signal, the rotation angle of the crankshaft from the reference position is continuously detected.
However, the ring gear is gradually worn by contact with the pinion upon start of engine, and accordingly the output voltage of the magnetic sensor may drop or the output voltage ratio may vary. In such a case, the detection precision of the rotation angle may also decrease.